After a catheter is inserted into the vascular system at an access site, it is advanced along large vessels to the vascular structure that requires treatment. Contrast agent is injected via the catheter and cathlab x-ray equipment records an angiographic sequence that shows the vessels when filled with contrast agent. The diagnostic angiogram acquisitions can be repeated with varying imager geometries. Diagnosis and intervention planning are based on such diagnostic angiograms.
During intervention, a flexible, partially or fully radio-opaque guidewire is advanced to the affected vascular structures (e.g. stenoses in coronaries, neurovascular aneurisms, or arterio-venous malformations). Fluoroscopic low-dose x-ray surveillance visualizes the guidewire and allows for the hand-eye-coordination of the interventionalist while advancing the guidewire. When positioned, the guidewire serves as rail to deliver interventional devices (e.g. balloons for dilation and stent delivery, detachable coils for aneurysm clotting). The delivery and deployment of the interventional devices is also fluoroscopy-controlled.
An overlay technique of the angiogram into the live images (referred to as roadmapping) may be utilized. In such procedures, the vessel structure itself is not visible during the intervention as it is not radio-opaque. Consequently, the navigation and precise positioning of guidewire and interventional devices is tedious, time-consuming, and requires additional contrast agent bursts to clarify the position of the devices relative to the relevant vessels. Due to scatter, both patient and medical staff are exposed to x-ray during the acquisition of diagnostic angiograms and interventional fluoroscopy. Navigation support is desired to reduce the intervention time and to enhance the positioning accuracy. Routinely, a static diagnostic angiogram acquired with a similar imager geometry is displayed next to the live interventional fluoroscopy. For the navigation of guidewire and devices within the vessels, a subjective visual fusion of the static angiogram and the live fluoroscopy is required. An improved context-rich visualization could give important support in navigation. As an approach, preprocessed angiograms can be overlaid onto the fluoroscopic image stream so that vessels and the interventional devices are synchronously displayed on one screen (cf. for example FIG. 1).
A navigation system can therefore help the cardiologists by providing a cardiac roadmap displayed next or overlaid on the live fluoroscopy pictures. Ideally, this cardiac roadmap represents the vessel network acquired during angiography, with the same cardiac phase than the current live image, and registered with respect to breathing movements and patient motions.
In WO 2004034329 A2, there is described a basic method for realizing cardiac roadmapping, relying on the extraction of the cardiac and respiratory cycles, and on the matching of those cycles between the angiogram images (in filled state) and the live images.
Roadmapping is a very important feature since it provides (hopefully) the accurate localisation of the intervention device with respect to the vessel anatomy (otherwise invisible during most of the PCI (Percutaneous Coronary Intervention) time).
Roadmapping is even more interesting in the case of cardiac interventions since the mental registration otherwise performed by the cardiologist between the angiogram (usually one selected image) and the dynamic fluoroscopy sequence is a tiring and inaccurate process.
However, the enhanced fluoroscopy sequence that contains the roadmapping mask that comes from the angiogram sequence suffers from several serious drawbacks.
It is quite impossible to overlay the full angiogram to the fluoroscopy image because this creates background mixings and all sorts of disagreeable visual effects. As a consequence, in practice, the cardiac roadmap is deduced from the angiogram through a segmentation process that extracts a mask which is assumed to be a good segmentation of the injected coronaries. Unfortunately, such a segmentation process is complex and often (if not always) produces a mask which is highly suboptimal (incomplete vessels or over segmentations, artefacts, temporal instability).
The other drawbacks are relative to fluoroscopy. The navigation image (a real-time fluoroscopy sequence) is very noisy, and it contains possibly strong breathing motion.